One source for speed benchmarks for P5X is in Burton's thread in this forum, in comparison to P5 Windows, and they indicated that it's about 3X faster on Windows machines. I haven't seen any comparison between P5X and P4Classic, other than subjective remarks that the Firefly renderer is slower than the P4 renderer, and that P5X bogs down frequently. My guess is that posing is about the same speed in P5X and P4Classic on the same machine, so the reasons to upgrade are the advanced renderer and shader functions.